The following abbreviations are utilized throughout this document:
3GPP Third Generation Partnership Project
AS Application Server
CM Connection Management
CSoLTE CS Services over LTE Radio Access
CS Circuit-Switched
DTM Dual Transfer Mode
eMSC-S evolved MSC Server
EPC Evolved Packet Core
EPS Evolved Packet System
E-UTRAN Evolved UTRAN
FDMA Frequency Division Multiple Access
GSM Global System for Mobile Communications
IASA Inter-Access Anchor
IMS IP Multimedia Subsystem
LAI Location Area Identifier
LTE Long Term Evolution
MME Mobility Management Entity
MSS Mobile Softswitch Solution
NAS Non Access Stratum
OFDM Orthogonal Frequency Division Multiplexing
PCRF Policy Charging Rule Function
PMSC Packet MSC
PCSC Packet CS Controller
PS Packet-Switched
RRC Radio Resource Control
SAE System Architecture Evolution
SAI Service Area Identifier
SCCP Signaling Connection Control Part
SC-FDMA Single Carrier Frequency Division Multiple Access
TA Tracking Area
UPE User Plane Entity
UTRAN Universal Terrestrial Radio Access Network
WCDMA Wideband Code Division Multiple Access
Mobile CS services based on GSM and WCDMA radio access are a world-wide success story and provide telecommunication services with a single subscription in almost all countries of the world. The number of CS subscribers is still growing rapidly, boosted by the rollout of mobile CS services in dense population countries such as India and China. This success story is furthermore extended by the evolution of the classical MSC architecture into a softswitch solution, which utilizes a packet transport infrastructure for mobile CS services.
Recently, the 3GPP work item “Evolved UTRA and UTRAN” (i.e., E-UTRAN, started in summer 2006) defined a Long-Term Evolution (LTE) concept that assures competitiveness of 3GPP-based access technology. It was preceded by an extensive evaluation phase of possible features and techniques in the RAN workgroups that concluded that the agreed system concepts can meet most of the requirements and no significant issue was identified in terms of feasibility.
LTE utilizes OFDM radio technology in the downlink and SC-FDMA for the uplink, allowing at least 100 Mbps peak data rate for downlink data rate and 50 Mbps for uplink data rate. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world.
FIG. 1 is a simplified block diagram of nodes in a System Architecture Evolution (SAE) Core Network (SAE CN) 11 and an LTE Radio Access Network (LTE RAN) 12. In parallel to the LTE RAN (E-UTRAN) standardization, 3GPP also drives an SAE work item to develop an evolved core network also called the Evolved Packet Core (EPC). The E-UTRAN and EPC together build up the Evolved Packet System (EPS). The SAE CN 11 is made up of core nodes, which may be further split into a Control Plane (Mobility Management Entity, MME) node 13 and a User Plane (SAE Gateway, SAE-GW) node 14. In the terminology currently used, the SAE-GW contains both User Plane Entity (UPE) and Inter-Access Anchor (IASA) functionality. The SAE-GW also has two different roles defined: Serving GW and Packet Data Network (PDN) GW. The term SAE-GW is used herein for both the Serving GW and the PDN GW. The MME 13 is connected to an eNodeB 15 via an S1-MME interface 16, and the SAE-GW 14 is connected to the eNodeB via the S1-U interface 17. The X2-UP and X2-CP interfaces between eNodeBs are not relevant to the present invention. The SAE architecture is further described in 3GPP TS 23.401 and 23.402.
Common to both LTE and SAE is that only a Packet Switched (PS) domain was initially to be specified, i.e., all services are to be supported via the PS domain. GSM (via DTM) and WCDMA, however, provide both PS and CS access simultaneously. Thus, if telephony services are to be deployed over LTE radio access, an IMS-based service engine is mandatory. It has been recently investigated how to use LTE/SAE as access technology to the existing Mobile Softswitch Solution (MSS) infrastructure. This work, referred to as “CS over LTE” (CSoLTE) or the longer name “CS domain services over evolved PS access,” is documented in 3GPP TR 23.879 and in 3GPP TS 23.272.
FIG. 2 is a simplified block diagram of a CSoLTE general architecture 20. A Packet MSC (PMSC) 21 serves both traditional 2G and 3G RANs 22 and the CSoLTE solutions through the LTE RAN 12. The PMSC contains two new logical functions: a Packet CS Controller (PCSC) 23 and an Interworking Unit (IWU) 24. In addition, there is an SGs interface 25 between the MME 13 and an MSC Server (MSC-S) 26. This interface is used for Paging and Mobility Management (MM) signaling to attach a mobile terminal 27 in the MSC-S based on, for example, SAE MM procedures performed between the terminal and the MME using similar principles as exists already for the Gs-interface between the MSC and SGSN in existing GSM and WCDMA networks and defined in 3GPP TS 29.016 and 29.018. The protocol used in the Gs-interface is called BSSAP+ and uses connectionless SCCP and normal MTP layers (or M3UA with SIGTRAN) in the existing implementations.